1- what is the endpoint being used by DrupalDirectoryManagerImpl the fetch user information?
There are actually 2 separate endpoints that are being used. The directory manager issues an authentication call and then a hashed authentication call as a fallback if the primary should fail. The endpoints are (drupal url)?q=joget/login and (drupal url)?q=joget/user. The Drupal url is a configurable property in your directory manager plugin.
2- are the sources for Drupal/Joget plugin available?
I'm going to come back and update this one later. I'm not sure if the plugins are configured to be pushed to our Github to be honest. Watch this space.
3- Is there any Drupal module i must install prior to using the above cited plugin?
Yes there is. This is probably the bit you're missing that's causing the error (i.e. the directory manager issuing an authentication call to an endpoint that drupal can't service because its lacking the modules). If you take a peek at ?Drupal, you'll see 2 elements that needs to be set up. The link to the module download should be there as well. I've submitted the module for inclusion into Drupal's public repository but they're taking their time with that. Once they finish their review, you should be able to download it directly from their spot but the link from ours will work just as well.
Hope that helps :)